The present disclosure relates to a thin film transistor, and a method of manufacturing the thin film transistor.
In many cases, thin film transistors (TFTs) have been used in a liquid crystal display panel as active elements for driving pixels. There are TFTs using amorphous silicons and TFTs using polycrystalline silicons as semiconductor. Polycrystalline silicons have larger electron mobilities compared with amorphous silicons. Therefore, in the TFTs using a polycrystalline silicon, a high speed action can be made. On the contrary, since amorphous silicons have smaller mobilities as compared with polycrystalline silicons, the TFTs using amorphous silicons can decrease leakage electric currents when not actuated.
JP 2012-114131 A discloses a TFT using properties of both of an amorphous silicon and a polycrystalline silicon. In this TFT, a gate electrode is formed on an insulating substrate; an insulating layer for covering the substrate and the gate electrode is formed; a polycrystalline silicon layer is formed on the insulating layer; an amorphous silicon layer is formed on the polycrystalline silicon layer; and a source electrode and a drain electrode are formed on the amorphous silicon layer. The polycrystalline silicon layer is formed by irradiating the amorphous silicon with a laser beam to change the amorphous silicon to the polycrystalline silicon after formation of the amorphous silicon. The polycrystalline silicon layer and the amorphous silicon layer function as a channel layer. TFT properties such as an electron mobility are intermediate properties between these in the case of a channel layer having a polycrystalline silicon and these in the case of a channel layer having an amorphous silicon.
Further, a technology of changing a part of an amorphous silicon to a polycrystalline silicon by partially irradiating the amorphous silicon with laser beam but not changing the whole of the amorphous silicon to the polycrystalline silicon has been developed. As compared with the case where the whole of the amorphous silicon is changed to the polycrystalline silicon, a leakage electric current of a TFT can be decreased by changing the part of the amorphous silicon layer to the polycrystalline silicon layer.